Datasheet

MAX17497A/MAX17497B
AC-DC and DC-DC Peak Current-Mode Converters
with Integrated Step-Down Regulator
16Maxim Integrated
The startup capacitor (C
START
) can be calculated as
follows:
GATE sw SSF
START IN
6
Q ft
C I µF
10
10


×
=+×




where I
IN
is the supply current drawn at the IN pin in mA,
Q
GATE
is the gate charge of the external MOSFET used
in nC, f
SW
is the switching frequency of the converter in
Hz, and t
SSF
is the soft-start time programmed for the
flyback converter in ms.
Resistors R
START
and R
ISRC
can be calculated as:
START
START
BEQ1
ISRC
V
RM
10
V
RM
70
= Ω
= Ω
The IN UVLO rising threshold of the MAX17497B is
set to 3.9V with hysteresis of 200mV, optimized for
low-voltage DC-DC applications down to 4.5V. For
applications where the input DC voltage is low enough
(e.g., 4.5V to 5.5V DC) so the power loss incurred to
supply the operating current of the MAX17497B can be
tolerated, the IN pin is directly connected to the DC input
(Figure 6). For higher DC input voltages (e.g., 16V to 32V
DC), a startup circuit (Figure 7) can be used to minimize
power dissipation in the startup circuit. In this start-
up scheme, the transistor (Q1)
supplies the switching
current until a bias winding NB comes up. The resistor
(R
Z
) can be calculated as:
Z INMIN
R 9 (V 6.3) k=× −Ω
where V
INMIN
is the minimum input DC voltage.
Programming the Soft-Start of the
Flyback/Boost Converter (SSF)
The devices’ soft-start period of the flyback/boost con-
verter can be programmed by selecting the value of
the capacitor connected from the SSF pin to GND. The
capacitor (C
SSF
) can be calculated as:
SSF SSF
C 8.13 t nF= ×
where t
SSF
is expressed in ms.
Figure 5. MAX17497A Current-Source-Based Startup Circuit
Figure 6. MAX17497B Typical Startup Circuit with IN
Connected Directly to DC Input
R
START
R
IN3
M
1
IN
Q1
R
ISRC
V
OUTF
V
DC
V
DC
V
OUTF
D1
IN
LXF
PGNDF
V
CC
C
VCC
C
START
D2
R
IN2
R
IN1
C
OUTF
MAX17497A
LDO
V
DC
V
OUTF
IN
LXF Np Ns
V
CC
D1
IN
C
IN
C
OUTF
C
VCC
MAX17497B
LDO